XiaoLei Hao

Alignment-Dependent Fluorescence Emission Induced by Tunnel Ionization of Carbon Dioxide from Lower-Lying Orbitals

The study of the ionization process of molecules in an intense infrared laser field is of paramount interest in strong-field physics and constitutes the foundation of imaging of molecular valence orbitals and attosecond science. We show measurement of alignment-dependent ionization probabilities of the lower-lying orbitals of the molecules by experimentally detecting the alignment dependence of fluorescence emission from tunnel ionized carbon dioxide molecules. The experimental measurements are compared with the theoretical calculations of the strong field approximation and molecular Ammosov-Delone-Krainov models. Our results demonstrate the feasibility of an all-optical approach for probing the ionization dynamics of lower-lying orbitals of molecules, which is until now still difficult to achieve by other techniques. Moreover, the deviation between the experimental and theoretical results indicates the incompleteness of current theoretical models for describing strong field ionization of molecules.

Quantum Orbits: A Powerful Concept in Laser-Atom Physics

Additional support for the concept of ”quantum orbits” is presented that emphasizes in particular the importance of ”long orbits” where the time between ionization and rescattering may be many periods of the laser field. Two examples are discussed, above-threshold ionization by an elliptically polarized laser field and intensity-dependent enhancements of certain spectral regions within the backscattering plateau, where we compare experimental data and the theoretical quantum-orbit simulations. In both cases, good agreement is obtained.

Laser-Induced Inelastic Diffraction from Strong-Field Double Ionization

In this Letter, we propose a novel laser-induced inelastic diffraction (LIID) scheme based on the intense-field-driven atomic nonsequential double ionization (NSDI) process and demonstrate that, with this LIID approach, the doubly differential cross sections (DDCSs) of the target ions, e.g., Ar^{+} and Xe^{+}, can be accurately extracted from the two-dimensional photoelectron momentum distributions in the NSDI process of the corresponding atoms. The extracted DDCSs exhibit a strong dependence on both the target and the laser intensity, in good agreement with calculated DDCSs from the scattering of free electrons. The LIID scheme may be extended to molecular systems and provides a promising approach for imaging of the gas-phase molecular dynamics induced by a strong laser field with unprecedented spatial and temporal resolution.

Effect of two-center interference on molecular ionization in multiphoton ionization regime

Using solution of the full three-dimensional time-dependent Schrödinger equation (TDSE) in prolate spheroidal coordinates, we investigate the orientation dependence of ionization of H2+ in near-infrared laser fields. It is found that, the ionization probability decreases as a function of the alignment angle in tunneling ionization regime, while it ascends with the increase of orientation angle in multiphoton ionization regime for the internuclear distance R=2 a.u. Furthermore, the result obtained by the length gauge strong-field approximation theory is in qualitative agreement with that calculated by the TDSE but the radiation gauge strong-field approximation and molecular ADK theories fail to reproduce the TDSE result. Analysis indicates that the above intriguing feature can be ascribed to the interference between the partial electron wave packets emitted from different molecular cores, which becomes evident at low laser intensity due to increased width of the initial mechanical momentum of the photoelectron at ionization moment. In addition, when the internuclear distance increases to R=4 a.u., the ionization yields decrease vs alignment angle in both tunneling and multiphoton regimes since the electron wavefunction of the 1σg orbit is more concentrated in the molecular axis than that of R=2 a.u.

Wavelength dependence of the Two Channels for Double Ionization of He in Intense Laser Field

In this paper, we investigate the double ionization process of He for the shakeoff (SO) and correlated energy-sharing (CES) channels in intense laser field based on the intense-field many body S-matrix theory (IMST). The wavelength dependence of the ionization rate from these two channels are analyzed and compared with each other. According to our calculation, the CES channel dominants at long wavelengths (300-780nm), while the SO channel dominants at short wavelengths (20-200nm). The more interesting thing occurs in the intermediate wavelength regime (200-300nm) where the contributions from the SO and CES channels are comparable and may compete with each other.

Long-Range Coulomb Effect in Intense Laser-Driven Photoelectron Dynamics

In strong field atomic physics community, long-range Coulomb interaction has for a long time been overlooked and its significant role in intense laser-driven photoelectron dynamics eluded experimental observations. Here we report an experimental investigation of the effect of long-range Coulomb potential on the dynamics of near-zero-momentum photoelectrons produced in photo-ionization process of noble gas atoms in intense midinfrared laser pulses. By exploring the dependence of photoelectron distributions near zero momentum on laser intensity and wavelength, we unambiguously demonstrate that the long-range tail of the Coulomb potential (i.e., up to several hundreds atomic units) plays an important role in determining the photoelectron dynamics after the pulse ends.

Quantitative identification of different strong-field ionization channels in the transition regime

We make a quantitative investigation on the tunneling and multi-photon channels in the transition regime from Keldysh parameter γ < 1 to γ > 1 by numerically solving the time-dependent Schrödinger equation (TDSE). A method is proposed to separate the contributions of those ionization channels based on the characteristics of the current. By analysing the dependence of the ionization rate on the Keldysh parameter γ, we identify a field independent transition point at γ ≈ 2, which is different from the well-accepted consensus of γ ≈ 1, from adiabatic to nonadiabatic regime.

Nonadiabatic Effect on the Rescattering Trajectories of Electrons in Strong Laser Field Ionization Process

The important features of the rescattering trajectories in strong field ionization process such as the cutoff of the return energy at 3.17UP and that of the final energy at 10UP are obtained, based on the adiabatic approximation in which the initial momentum of the electron is assumed to be zero. We theoretically study the nonadiabatic effect by assuming a nonzero initial momentum on the rescattering trajectories based on the semiclassical simpleman model. We show that the nonzero initial momentum will modify both the maximal return energy at collision and the final energy after backward scattering, but in different ways for odd and even number of return trajectories. The energies are increased for even number of returns but are decreased for odd number of returns when the nonzero (positive or negative) initial momentum is applied.